feat: parallel functional tests (~3x faster CI)

- Split 69 functional tests into 4 groups for parallel execution
- Add check-functional-parallel target to Makefile.am
- Fix VPATH builds with $(srcdir)/tests path
- Add test failure detection verification in CI
- Update Dockerfiles with parallel make -j$(nproc)
- Add --depth 1 for faster git clone in Dockerfiles

Dockerfile.debian

@@ -3,11 +3,13 @@ FROM debian:testing
 
 ENV DEBIAN_FRONTEND="noninteractive"
 ENV TERM=xterm
+ENV CC="ccache gcc"
 
 RUN apt-get update && apt-get install -y --no-install-recommends \
   autoconf \
   autoconf-archive \
   automake \
+  ccache \
   gcc \
   git \
   libcmocka-dev \